High strength steel sheet and method for manufacturing the same

ABSTRACT

A high strength steel sheet having composition includes, on a percent by mass basis, C: 0.17% to 0.73%; Si: 3.0% or less; Mn: 0.5% to 3.0%; P: 0.1% or less; S: 0.07% or less; Al: 3.0% or less; and N: 0.010% or less, satisfies Si+Al≧0.7%, and the remainder includes Fe and incidental impurities, with a microstructure that has an area percentage of a total amount of lower bainite and whole martensite 10% to 90% relative to the whole steel sheet microstructure, an amount of retained austenite is 5% to 50%, an area percentage of bainitic ferrite in upper bainite is 5% or more relative to the whole steel sheet microstructure, as-quenched martensite is 75% or less of the total amount of lower bainite and whole martensite, and an area percentage of polygonal ferrite is 10% or less relative to the whole steel sheet microstructure, an average amount of C in retained austenite is 0.70% or more, and tensile strength is 980 MPa or more.

RELATED APPLICATIONS

This is a §371 of International Application No. PCT/JP2009/065981, withan interational filing date of Sep. 8, 2009 (WO 2010/030021 A1,published Mar. 18, 2010), which is based on Japanese Patent ApplicationNo. 2008-232437, filed Sep. 10, 2008, the subject matter of which isincorporated by reference.

TECHNICAL FIELD

This disclosure relates to a high strength steel sheet used inindustrial fields of automobile, electric apparatus, and the like andwhich has excellent workability, especially elongation andstretch-flangeability, and a tensile strength (TS) of 980 MPa or more,and a method for manufacturing the same.

BACKGROUND

In recent years, enhancement of fuel economy of the automobile hasbecome an important issue from the viewpoint of global environmentalconservation. Consequently, there is an active movement afoot to reducethe thickness of car components through increases in strength of carbody materials to reduce the weight of a car body itself.

In general, to increase the strength of a steel sheet, it is necessaryto increase the proportion of a hard phase, e.g., martensite or bainite,relative to the whole microstructure of the steel sheet. However, theincrease in strength of the steel sheet through the increase inproportion of the hard phase causes a reduction in workability.Therefore, development of a steel sheet having high strength andexcellent workability in combination has been desired. Various complexmicrostructure steel sheets, e.g., a ferrite-martensite double phasesteel (DP steel) and a TRIP steel taking the advantage of thetransformation induced plasticity of retained austenite, have beendeveloped.

In the case where the proportion of the hard phase in the complexmicrostructure steel sheet increases, the workability of the steel sheetis affected by the workability of the hard phase significantly. This isbecause in the case where the proportion of the hard phase is small andthat of soft polygonal ferrite is large, the deformability of polygonalferrite is predominant over the workability of the steel sheet, and evenin the case where the workability of the hard phase is inadequate, theworkability, e.g., elongation, is ensured, whereas in the case where theproportion of the hard phase is large, deformability of the hard phaseitself rather than deformation of polygonal ferrite exerts an influencedirectly on formability of the steel sheet. Therefore, if theworkability of the hard phase itself is inadequate, deterioration of theworkability of the steel sheet becomes significant.

Consequently, as for a cold rolled steel sheet, after conducting a heattreatment to adjust the amount of polygonal ferrite generated duringannealing and cooling thereafter, the steel sheet is water-quenched togenerate martensite, the temperature is raised again, and the steelsheet is kept at high temperatures so that martensite is tempered,carbides are generated in martensite, which is a hard phase, andthereby, workability of martensite is improved. However, suchquenching•tempering of martensite needs a specific production facility,for example, a continuous annealing facility having a water quenchingfunction. Therefore, in the case where a common facility is used, inwhich after the steel sheet is water-quenched, it is not possible toraise the temperature again and maintain high temperatures, the strengthof the steel sheet can be increased but the workability of martensite,which is a hard phase, cannot be improved.

Furthermore, as for a steel sheet in which the hard phase is other thanmartensite, there is a steel sheet in which a primary phase is polygonalferrite, a hard phase is bainite and pearlite, and carbides aregenerated in such bainite and pearlite serving as the hard phase. Thissteel sheet exhibits improved workability not only by polygonal ferrite,but also by generating carbides in the hard phase to improve theworkability of the hard phase in itself and, in particular, animprovement of the stretch-flangeability is intended. However, since theprimary phase is polygonal ferrite, it is difficult to allow an increasein strength to 980 MPa or more in terms of tensile strength (TS) and theworkability to become mutually compatible. In this connection, even whenthe workability of the hard phase itself is improved by generatingcarbides in the hard phase, the level of workability is inferior to thatof polygonal ferrite. Therefore, if the amount of polygonal ferrite isreduced to increase the strength to 980 MPa or more in terms of tensilestrength (TS), adequate workability cannot be obtained.

Japanese Unexamined Patent Application Publication No. 4-235253 proposesa high strength steel sheet having excellent bendability and impactcharacteristic, wherein alloy components are specified and the steelmicrostructure is fine uniform bainite including retained austenite.

Japanese Unexamined Patent Application Publication No. 2004-76114proposes a complex microstructure steel sheet having excellent bakehardenability, wherein predetermined alloy components are specified, thesteel microstructure is bainite including retained austenite, and theamount of retained austenite in the bainite is specified.

Japanese Unexamined Patent Application Publication No. 11-256273proposes a complex microstructure steel sheet having excellent impactresistance, wherein predetermined alloy components are specified, thesteel microstructure is specified in such a way that bainite includingretained austenite is 90% or more in terms of area percentage and theamount of austenite in the bainite is 1% or more, and 15% or less, andthe hardness (HV) of the bainite is specified.

However, the above-described steel sheets have problems as describedbelow.

Regarding the component composition described in JP '253, it isdifficult to ensure the amount of stable retained austenite which exertsa TRIP effect in a high strain region in the case where a strain isapplied to a steel sheet. Therefore, although bendability is obtained,elongation is low when plasticity becomes unstable, and punchstretchability is poor.

Regarding the steel sheet described in JP '114, bake hardenability isobtained. However, in the case where an increase in strength is intendedin such a way that the tensile strength (TS) becomes 980 MPa or more, orfurthermore, 1,050 MPa or more, it is difficult to ensure the strengthor workability, e.g., elongation and stretch-flangeability, when thestrength increases because the microstructure primarily contains bainiteand, furthermore, ferrite while martensite is minimized.

The steel sheet described in JP '273 is for the purpose of improving theimpact resistance and the microstructure contains bainite having ahardness of HV 250 or less as a primary phase, specifically at a contentexceeding 90%. Therefore, it is difficult to make the tensile strength(TS) 980 MPa or more.

It could therefore be helpful to provide a high strength steel sheethaving excellent workability, especially the elongation and thestretch-flangeability, and a tensile strength (TS) of 980 MPa or more,as well as an advantageous method for manufacturing the same.

SUMMARY

Our high strength steel sheets include a steel sheet in whichgalvanizing or galvannealing is applied to a surface of the steel sheet.

Excellent workability refers to a value of TS×T.EL satisfying 20,000MPa·% or more and a value of TS×λ satisfying 25,000 MPa·% or more. Inthis regard, TS represents tensile strength (MPa), T.EL represents totalelongation (%), and λ represents hole-expansion limit (%).

We conducted intensive research on the component composition andmicrostructure of steel sheets. We found that strength was increasedthrough the use of a lower bainite microstructure and/or a martensitemicrostructure, stable retained austenite which was advantageous toobtain a TRIP effect, was ensured through the use of upper bainitetransformation while the C content was increased in such a way that theamount of C in the steel sheet became 0.17% or more, a portion of themartensite was converted to tempered martensite and, thereby, a highstrength steel sheet having excellent workability, especially a balancebetween the strength and the elongation and a balance between thestrength and the stretch-flangeability in combination, and a tensilestrength of 980 MPa or more was obtained.

We thus provide:

-   -   1. A high strength steel sheet characterized by having a        composition containing, on a percent by mass basis,        -   C: 0.17% or more, and 0.73% or less,        -   Si: 3.0% or less,        -   Mn: 0.5% or more, and 3.0% or less,        -   P: 0.1% or less,        -   S: 0.07% or less,        -   Al: 3.0% or less, and        -   N: 0.010% or less,    -   while it is satisfied that Si+Al is 0.7% or more, and the        remainder includes Fe and incidental impurities,        -   wherein regarding the steel sheet microstructure, it is            satisfied that the area percentage of a total amount of            lower bainite and whole martensite is 10% or more, and 90%            or less relative to the whole steel sheet microstructure,            the amount of retained austenite is 5% or more, and 50% or            less, the area percentage of bainitic ferrite in upper            bainite is 5% or more relative to the whole steel sheet            microstructure, as-quenched martensite is 75% or less of the            above-described total amount of lower bainite and whole            martensite, and the area percentage of polygonal ferrite is            10% or less (including 0%) relative to the whole steel sheet            microstructure, the average amount of C in the            above-described retained austenite is 0.70% or more, and the            tensile strength is 980 MPa or more.    -   2. The high strength steel sheet according to the        above-described item 1, characterized in that the        above-described steel sheet further contains at least one type        of element selected from, on a percent by mass basis,        -   Cr: 0.05% or more, and 5.0% or less,        -   V: 0.005% or more, and 1.0% or less, and        -   Mo: 0.005% or more, and 0.5% or less.    -   3. The high strength steel sheet according to the        above-described item 1 or item 2, characterized in that the        above-described steel sheet further contains at least one type        of element selected from, on a percent by mass basis,        -   Ti: 0.01% or more, and 0.1% or less and        -   Nb: 0.01% or more, and 0.1% or less.    -   4. The high strength steel sheet according to any one of the        above-described items 1 to 3, characterized in that the        above-described steel sheet further contains, on a percent by        mass basis,        -   B: 0.0003% or more, and 0.0050% or less.    -   5. The high strength steel sheet according to any one of the        above-described items 1 to 4, characterized in that the        above-described steel sheet further contains at least one type        of element selected from, on a percent by mass basis,        -   Ni: 0.05% or more, and 2.0% or less, and        -   Cu: 0.05% or more, and 2.0% or less.    -   6. The high strength steel sheet according to any one of the        above-described items 1 to 5, characterized in that the        above-described steel sheet further contains at least one type        of element selected from, on a percent by mass basis,        -   Ca: 0.001% or more, and 0.005% or less, and        -   REM: 0.001% or more, and 0.005% or less.    -   7. A high strength steel sheet characterized by including a        galvanized layer or a galvannealed layer on a surface of the        steel sheet according to any one of the above-described items 1        to 6.    -   8. A method for manufacturing a high strength steel sheet,        characterized by including the steps of hot-rolling a billet        having a component composition according to any one of the        above-described items 1 to 6, conducting cold-rolling to produce        a cold-rolled steel sheet, annealing the resulting cold-rolled        steel sheet for 15 seconds or more, and 600 seconds or less in        an austenite single phase region and, thereafter, conducting        cooling to a cooling termination temperature: T° C. determined        in a first temperature range of 350° C. or higher, and 490° C.        or lower, wherein cooling to at least 550° C. is conducted while        the average cooling rate is controlled at 5° C./s or more,        subsequently, maintenance is conducted in the first temperature        range for 15 seconds or more, and 1,000 seconds or less and,        then, maintenance is conducted in a second temperature range of        200° C. or higher, and 350° C. or lower for 15 seconds or more,        and 1,000 seconds or less.    -   9. The method for manufacturing a high strength steel sheet        according to the above-described item 8, characterized in that a        galvanizing treatment or a galvannealing treatment is applied        during cooling to the above-described cooling termination        temperature: T° C. or in the above-described first temperature        range.

A high strength steel sheet having excellent workability, especially theelongation and the stretch-flangeability, and a tensile strength (TS) of980 MPa or more, as well as an advantageous method for manufacturing thesame can be provided. Therefore, the utility value in the industrialfields of automobiles, electric, and the like is very large, and inparticular, the usefulness in weight reduction of an automobile body issignificant.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram showing a temperature pattern of a heat treatment inour manufacturing method.

DETAILED DESCRIPTION

Initially, the reason for limitation of the steel sheet microstructurein such a way that described above will be described. Hereafter, “areapercentage” refers to an area percentage relative to the whole steelsheet microstructure.

Area Percentage of Total Amount of Lower Bainite and Whole Martensite:10% or more, and 90% or Less

Lower bainite and martensite are microstructures necessary to increasethe strength of the steel sheet. If the area percentage of a totalamount of lower bainite and whole martensite is less than 10%, the steelsheet does not satisfy the tensile strength (TS) of 980 MPa or more. Onthe other hand, if the total amount of lower bainite and wholemartensite exceeds 90%, the upper bainite is reduced and, as a result,stable retained austenite, in which C is concentrated, cannot beensured. Consequently, a problem occurs in that the workability, e.g.,elongation, deteriorates. Therefore, the area percentage of the totalamount of lower bainite and whole martensite is 10% or more, and 90% orless. A preferable range is 20% or more, and 80% or less. A morepreferable range is 30% or more, and 70% or less.

Proportion of As-Quenched Martensite in Total Amount of Lower Bainiteand Whole Martensite: 75% or Less

If the proportion of as-quenched martensite in the martensite exceeds75% of the total amount of lower bainite and whole martensite present inthe steel sheet, the tensile strength becomes 980 MPa or more, but thestretch-flangeability is poor. The as-quenched martensite is very hard,and deformability of the as-quenched martensite in itself is very low.Therefore, workability, especially stretch-flangeability, of the steelsheet deteriorates significantly. Furthermore, since the difference inhardness between the as-quenched martensite and the upper bainite issignificantly large, if the amount of as-quenched martensite is large,the interface between the as-quenched martensite and upper bainiteincreases. Consequently, fine voids are generated at the interfacebetween the as-quenched martensite and the upper bainite during punchingor the like, and in stretch-flange forming conducted after the punching,voids are coupled to each other so that cracking develops easily and,thereby, stretch-flangeability deteriorates. Therefore, the proportionof as-quenched martensite in the martensite is 75% or less relative tothe total amount of lower bainite and whole martensite present in thesteel sheet. Preferably, the proportion is 50% or less. In this regard,the as-quenched martensite is a microstructure in which no carbide isdetected in the martensite and can be observed with SEM. Amount ofretained austenite: 5% or more, and 50% or less

The retained austenite undergoes martensitic transformation through aTRIP effect during working and, thereby, strain dispersive power isenhanced to improve elongation.

Retained austenite in which the amount of concentrated C is increased,is formed in the upper bainite through the use of upper bainitetransformation. As a result, retained austenite capable of making theTRIP effect apparent even in a high strain region during working can beobtained. In the case where such retained austenite and martensite arepresent in combination and used, good workability is obtained even in ahigh strength region in which the tensile strength (TS) is 980 MPa ormore. Specifically, the value of TS×T.El can be 20,000 MPa·% or more,and a steel sheet having an excellent balance between strength andelongation can be obtained.

Since the retained austenite in the upper bainite is formed betweenlaths of bainitic ferrite in the upper bainite and distributes finely,large amounts of measurement at high magnification is necessary fordetermination of the amount (area percentage) thereof throughmicrostructure observation, and it is difficult to quantify accurately.However, the amount of retained austenite formed between laths of thebainitic ferrite is an amount corresponding to the amount of formedbainitic ferrite to some extent. We found that an adequate TRIP effectwas able to be obtained and the tensile strength (TS) of 980 MPa or moreand TS×T.El of 20,000 MPa·% or more were able to be achieved if the areapercentage of bainitic ferrite in the upper bainite was 5% or more, andthe amount of retained austenite determined by an intensity measurementwith X-ray diffraction (XRD), which was a previously employed techniqueto measure the amount of retained austenite, specifically, an X-raydiffraction intensity ratio of ferrite to austenite, was 5% or more. Inthis regard, we ascertained that the amount of retained austenitedetermined by the previously employed technique to measure the amount ofretained austenite is equivalent to the area percentage of retainedaustenite relative to the whole steel sheet microstructure.

In the case where the amount of retained austenite is less than 5%, anadequate TRIP effect is not obtained. On the other hand, if the amountexceeds 50%, hard martensite generated after the TRIP effect is madeapparent becomes excessive, deterioration of tenacity and the likebecome problems. Therefore, the amount of retained austenite is withinthe range of 5% or more, and 50% or less. The range is preferably morethan 5%, and more preferably within the range of 10% or more, and 45% orless. The range is further preferably within the range of 15% or more,and 40% or less.

Average Amount of C in Retained Austenite: 0.70% or More

Regarding a high strength steel sheet having a tensile strength (TS) of980 MPa to 2.5 GPa class, to obtain excellent workability through theuse of the TRIP effect, the amount of C in the retained austenite isimportant. C is concentrated into the retained austenite formed betweenlaths of bainitic ferrite in the upper bainite. It is difficult toaccurately evaluate the amount of C concentrated into the retainedaustenite between the laths. However, we found that excellentworkability was obtained when the average amount of C in the retainedaustenite determined from the amount of shift of a diffraction peak inthe X-ray diffraction (XRD), which was a previously employed method formeasuring the average amount of C in the retained austenite (an averageof the amount of C in the retained austenite), was 0.70% or more.

In the case where the average amount of C in the retained austenite isless than 0.70%, martensitic transformation occurs in a low strainregion during working so that the TRIP effect in a high strain region toimprove the workability is not obtained. Therefore, the average amountof C in the retained austenite is 0.70% or more. The amount ispreferably 0.90% or more. On the other hand, if the average amount of Cin the retained austenite exceeds 2.00%, the retained austenite becomesexcessively stable, martensitic transformation does not occur duringworking, and the TRIP effect is not apparent so that elongationdeteriorates. Therefore, it is preferable that the average amount of Cin the retained austenite is 2.00% or less. More preferably, the averageamount is 1.50% or less.

Area Percentage of Bainitic Ferrite in Upper Bainite: 5% or More

Generation of bainitic ferrite due to upper bainite transformation isnecessary to concentrate C in untransformed austenite to obtain retainedaustenite, which makes the TRIP effect apparent in a high strain regionduring working and which enhances strain resolution. The transformationfrom austenite to bainite occurs over a wide temperature range of about150° C. to 550° C., and bainite generated in this temperature rangeinclude various types. In many cases in the previous technology, suchvarious types of bainite has been specified as bainite simply. However,to obtain the desired workability, it is necessary that the bainitemicrostructure is specified clearly. Therefore, the upper bainite andthe lower bainite are defined as described below.

Upper bainite is characterized in that lath-shaped bainitic ferrite andretained austenite and/or carbides present between bainitic ferrite areincluded and fine carbides regularly arranged in the lath-shapedbainitic ferrite are not present. On the other hand, lower bainite ischaracterized in that lath-shaped bainitic ferrite and retainedaustenite and/or carbides present between bainitic ferrite are included,as is common to upper bainite, and in the lower bainite, fine carbidesregularly arranged in the lath-shaped bainitic ferrite are present.

That is, the upper bainite and the lower bainite are distinguished onthe basis of presence or absence of fine carbides regularly arranged inthe bainitic ferrite. The above-described difference in the generationstate of carbides in the bainitic ferrite exerts a significant influenceon concentration of C into the retained austenite. That is, in the casewhere the area percentage of bainitic ferrite in the upper bainite isless than 5%, even when bainite transformation proceeds, the amount of Cformed into carbides in the bainitic ferrite increases. As a result, theamount of concentration of C into the retained austenite present betweenlaths decreases, and a problem occurs in that the amount of retainedaustenite, which exerts the TRIP effect in a high strain region duringworking, decreases. Therefore, it is necessary that the area percentageof bainitic ferrite in the upper bainite is 5% or more in terms of areapercentage relative to the whole steel sheet microstructure. On theother hand, if the area percentage of bainitic ferrite in the upperbainite exceeds 85% relative to the whole steel sheet microstructure, itmay become difficult to ensure the strength. Consequently, it ispreferable that the area percentage is 85% or less.

Area Percentage of Polygonal Ferrite: 10% or Less (Including 0%)

If the area percentage of polygonal ferrite exceeds 10%, it becomesdifficult to satisfy the tensile strength (TS): 980 MPa or more and, atthe same time, strain is concentrated on soft polygonal ferrite presenttogether in the hard microstructure during working so that crackingoccurs easily during working. As a result, the desired workability isnot obtained. If the area percentage of the polygonal ferrite is 10% orless, even when the polygonal ferrite is present, a state in which asmall amount of polygonal ferrite is discretely dispersed in a hardphase is brought about, concentration of strain can be suppressed, anddeterioration of workability can be avoided. Therefore, the areapercentage of the polygonal ferrite is 10% or less. The area percentageis preferably 5% or less, further preferably 3% or less, and may be 0%.

The hardness of the hardest microstructure in the steel sheetmicrostructure is HV ≦800. That is, in the case where as-quenchedmartensite is not present in the steel sheet, any one of temperedmartensite, lower bainite, and upper bainite becomes the hardest phase.All of these microstructures are phases which become HV ≦800.Alternatively, in the case where as-quenched martensite is present, theas-quenched martensite becomes the hardest microstructure. Regarding theas-quenched martensite, the hardness becomes HV ≦800, a significantlyhard martensite exhibiting HV >800 is not present, and goodstretch-flangeability can be ensured.

The steel sheet may include pearlite, Widmanstaetten ferrite, and lowerbainite as the remainder microstructure. In that case, it is preferablethat the allowable content of the remainder microstructure is 20% orless in terms of area percentage. More preferably, the allowable contentis 10% or less.

The basic configuration of the steel sheet microstructure of the highstrength steel sheet is as described above, and the followingconfiguration may be added as necessary.

Next, the reason for limitation of component composition of the steelsheet in such a way that described above will be described. In thisconnection, % hereafter representing the following component compositionrefers to percent by mass.

C: 0.17% or More, and 0.73% or Less

The element C is an indispensable element to increase the strength ofthe steel sheet and ensure the amount of stable retained austenite, andan element necessary to ensure the amount of martensite and retainaustenite at room temperature. If the amount of C is less than 0.17%, itis difficult to ensure the strength and workability of the steel sheet.On the other hand, if the amount of C exceeds 0.73%, hardening of weldedand heat-affected zones is significant so that weldability deteriorates.Therefore, the amount of C is within the range of 0.17% or more, and0.73% or less. The range is preferably more than 0.20% and 0.48% orless, and further preferably 0.25% or more.

Si: 3.0% or Less (Including 0%)

The element Si contributes to an improvement in the strength of steel bystrengthening through solid solution. However, if the amount of Siexceeds 3.0%, an increase in the amount of solid solution into thepolygonal ferrite and the bainitic ferrite causes deterioration ofworkability and tenacity, and causes deterioration of surfacecharacteristics due to occurrence of red scale and the like anddeterioration of wettability and adhesion of the coating in the casewhere hot dipping is applied. Therefore, the amount of Si is 3.0% orless. The amount is preferably 2.6% or less. The amount is furtherpreferably 2.2% or less.

Moreover, Si is an element useful for suppressing generation of carbidesand facilitating generation of retained austenite. Therefore, it ispreferable that the amount of Si is 0.5% or more. However, in the casewhere generation of carbides is suppressed by merely Al, Si is notnecessarily added, and amount of Si may be 0%.

Mn: 0.5% or More, and 3.0% or Less

The element Mn is useful for strengthening steel. If the amount of Mn isless than 0.5%, carbides are deposited in a temperature range higherthan the temperature, at which bainite and martensite are generated,during cooling after annealing. Consequently, it is not possible toensure the amount of hard phase, which contributes to strengthening ofsteel. On the other hand, the amount of Mn exceeding 3.0% causesdeterioration of castability and the like. Therefore, the amount of Mnis 0.5% or more and 3.0% or less. The range is preferably 1.5% or moreand 2.5% or less.

P: 0.1% or Less

The element P is useful for strengthening steel. If the amount of Pexceeds 0.1%, the impact resistance deteriorates due to embrittlementbased on grain boundary segregation, and in the case where galvannealingis applied to a steel sheet, the alloying rate is reduced significantly.Therefore, the amount of P is 0.1% or less. The amount is preferably0.05% or less. In this connection, it is preferable that the amount of Pis reduced. However, reduction to less than 0.005% causes a significantincrease in cost. Therefore, it is preferable that the lower limitthereof is about 0.005%.

S: 0.07% or Less

The element S generates MnS to become an inclusion and causesdeterioration of the impact resistance and cracking along a metal flowof welded zones. Therefore, it is preferable that the amount of S isminimized. However, since excessive reduction in the amount of S causesan increase in production cost, the amount of S is 0.07% or less.Preferably, the amount is 0.05% or less, and more preferably 0.01% orless. In this connection, reduction of S to less than 0.0005% isattended with a significant increase in production cost. Therefore, thelower limit thereof is about 0.0005% from the viewpoint of theproduction cost.

Al: 3.0% or Less

The element Al is useful for strengthening steel and, in addition, is auseful element which is added as a deoxidizing agent in a steel makingprocess. If the amount of Al exceeds 3.0%, inclusion in a steel sheetincreases and elongation deteriorates. Therefore, the amount of Al is3.0% or less. The amount is preferably 2.0% or less.

Moreover, Al is an element useful for suppressing generation of carbidesand facilitating generation of retained austenite. Furthermore, it ispreferable that the amount of Al is 0.001% or more to obtain adeoxidation effect, and more preferably 0.005% or more. In this regard,the amount of Al is the amount of Al contained in the steel sheet afterdeoxidation.

N: 0.010% or Less

The element N causes maximum deterioration of the aging resistance ofsteel and is preferably minimized. If the amount of N exceeds 0.010%,deterioration of the aging resistance becomes significant and,therefore, the amount of N is 0.010% or less. In this connection,reduction of N to less than 0.001% causes a significant increase inproduction cost so that the lower limit thereof is about 0.001% from theviewpoint of production cost.

Up to this point, the basic components have been described. However,only satisfaction of the above-described component ranges is notadequate, and it is necessary that the following formula is satisfied.

Si+Al≧0.7%

As described above, both Si and Al are elements useful for suppressinggeneration of carbides and facilitating generation of retainedaustenite. Regarding suppression of generation of carbides, an effect isexerted by containing Si or Al alone, but it is necessary to satisfythat a total of the amount of Si and Al is 0.7% or more. In thisconnection, the amount of Al in the above-described formula is theamount of Al contained in the steel sheet after deoxidation.

In addition, the components described below can be containedappropriately besides the above-described basic components.

At Least One Type Selected from Cr: 0.05% or More, and 5.0% or Less, V:0.005% or More, and 1.0% or Less, and Mo: 0.005% or More, and 0.5% orLess

The elements Cr, V, and Mo function to suppress generation of pearliteduring cooling from an annealing temperature. The effect thereof isobtained at Cr: 0.05% or more, V: 0.005% or more, and Mo: 0.005% ormore. On the other hand, if Cr: 5.0%, V: 1.0%, and Mo: 0.5% areexceeded, the amount of hard martensite becomes too large, and thestrength becomes high more than necessary. Therefore, in the case whereCr, V, and Mo are contained, the ranges are Cr: 0.05% or more and 5.0%or less, V: 0.005% or more and 1.0% or less, and Mo: 0.005% or more and0.5% or less.

At Least One Type Selected from Ti: 0.01% or More, and 0.1% or Less andNb: 0.01% or More, and 0.1% or Less

The elements Ti and Nb are useful for strengthening steel throughdeposition, and the effect thereof is obtained when the individualcontents are 0.01% or more. On the other hand, if the individualcontents exceed 0.1%, workability and shape fixability deteriorate.Therefore, in the case where Ti and Nb are contained, the ranges are Ti:0.01% or more and 0.1% or less and Nb: 0.01% or more and 0.1% or less.

B: 0.0003% or More, and 0.0050% or Less

The element B is useful for suppressing generation•growth of ferritefrom austenite grain boundaries. The effect thereof is obtained when thecontent is 0.0003% or more. On the other hand, if the content exceeds0.0050%, workability deteriorates. Therefore, in the case where B iscontained, the range is B: 0.0003% or more and 0.0050% or less.

At Least One Type Selected from Ni: 0.05% or More, and 2.0% or Less andCu: 0.05% or More, and 2.0% or Less

The elements Ni and Cu are useful for strengthening steel. Furthermore,in the case where galvanizing or galvannealing is applied to a steelsheet, internal oxidation of a steel sheet surface layer portion isfacilitated and, thereby, adhesion of the coating is improved. Theseeffects are obtained when individual contents are 0.05% or more. On theother hand, if the individual contents exceed 2.0%, the workability ofthe steel sheet deteriorates. Therefore, in the case where Ni and Cu arecontained, the ranges are Ni: 0.05% or more and 2.0% or less and Cu:0.05% or more and 2.0% or less.

At Least One Type Selected from Ca: 0.001% or More, and 0.005% or Lessand REM: 0.001% or More, and 0.005% or Less

The elements Ca and REM are useful for spheroidizing the shape ofsulfides and improve the adverse effect of sulfides onstretch-flangeability. The effects thereof are obtained when individualcontents are 0.001% or more. On the other hand, if the individualcontents exceed 0.005%, increases of inclusion and the like are invitedto cause surface defects, internal defects, and the like. Therefore, inthe case where Ca and REM are contained, the ranges are Ca: 0.001% ormore and 0.005% or less and REM: 0.001% or more and 0.005% or less.

The components other than those described above are Fe and incidentalimpurities. However, components other than those described above may becontained within the bounds of not impairing the effects of our steelsheets.

Next, a method for manufacturing a high strength steel sheet will bedescribed.

After a billet adjusted to have the above-described favorable componentcomposition is produced, hot-rolling is conducted and, then,cold-rolling is conducted to produce a cold-rolled steel sheet. Thesetreatments are not specifically limited and may be conducted followingusual methods.

Favorable production conditions are as described below. After the billetis heated to a temperature within the range of 1,000° C. or higher and1,300° C. or lower, the hot rolling is terminated in a temperature rangeof 870° C. or higher and 950° C. or lower. The resulting hot-rolledsteel sheet is taken up in a temperature range of 350° C. or higher and720° C. or lower. Subsequently, the hot-rolled steel sheet is pickledand, thereafter, cold-rolling is conducted at a reduction ratio withinthe range of 40% or more and 90% or less to produce a cold-rolled steelsheet.

In this connection, it is assumed that the steel sheet is producedthrough usual individual steps of steel making, casting, hot rolling,pickling, and cold rolling. However, for example, production may beconducted through thin slab casting or strip casting while a part of oran entire hot rolling step is omitted.

A heat treatment shown in FIG. 1 is applied to the resulting cold-rolledsteel sheet. The explanation will be conducted below with reference toFIG. 1.

Annealing is conducted for 15 seconds or more and 600 seconds or less inan austenite single phase region. The steel sheet contains upperbainite, lower bainite, and martensite, which are transformed fromuntransformed austenite in a relatively low temperature range of 350° C.or higher and 490° C. or lower, as primary phases. Therefore, it ispreferable that polygonal ferrite is minimized and annealing in anaustenite single phase region is required. The annealing temperature isnot specifically limited insofar as it is in the austenite single phaseregion. If the annealing temperature exceeds 1,000° C., growth ofaustenite grains is significant, coarser configuration phases aregenerated by downstream cooling, and tenacity and the like deteriorate.On the other hand, in the case where the annealing temperature is lowerthan A₃ point (austenite transformation point), polygonal ferrite hasalready been generated in an annealing stage, and it becomes necessarythat a temperature range of 500° C. or more is cooled very rapidly tosuppress growth of polygonal ferrite during cooling. Therefore, it isnecessary that the annealing temperature is the A₃ point or higher and,preferably, 1,000° C. or lower.

Furthermore, if the annealing time is less than 15 seconds, in somecases, reverse transformation to austenite does not proceed adequatelyor carbides in the steel sheet are not dissolved adequately. On theother hand, if the annealing time exceeds 600 seconds, an increase incost is invited along with high energy consumption. Therefore, theannealing time is within the range of 15 seconds or more, and 600seconds or less. Preferably, the annealing time is within the range of60 seconds or more, and 500 seconds or less. The A₃ point can becalculated on the basis of

A₃ point (° C.)=910−203×[C %]½+44.7×[Si %]−30×[Mn %]+700×[P %]+130×[Al%]−15.2×[Ni %]−11×[Cr %]−20×[Cu %]+31.5×[Mo %]+104×[V %]+400×[Ti %].

In this connection, [X %] represents percent by mass of componentelement X of the steel sheet.

The cold-rolled steel sheet after annealing is cooled to a coolingtermination temperature: T° C. determined in a first temperature rangeof 350° C. or higher and 490° C. or lower, wherein cooling to at least550° C. is conducted while the average cooling rate is controlled at 5°C./s or more. In the case where the average cooling rate is less than 5°C./s, excessive generation and growth of polygonal ferrite, depositionof pearlite and the like occur so that a desired steel sheetmicrostructure is not obtained. Therefore, the average cooling rate fromthe annealing temperature to the first temperature range is 5° C./s ormore. Preferably, the average cooling rate is 10° C./s or more. Theupper limit of the average cooling rate is not specifically limitedinsofar as variations do not occur in the cooling terminationtemperature. If the average cooling rate exceeds 100° C./s, variationsin microstructure in a longitudinal direction and a sheet widthdirection of a steel sheet becomes large significantly. Therefore, 100°C./s or less is preferable.

The steel sheet cooled to 550° C. is cooled succeedingly to the coolingtermination temperature: T° C. The rate of cooling of the steel sheet inthe temperature range of T° C. or higher and 550° C. or lower is notspecifically limited except that a maintenance time in the firstmaintenance temperature range is 15 seconds or more and 1,000 seconds orless. However, in the case where the steel sheet is cooled at a too lowrate, carbides are generated from untransformed austenite and, thereby,there is a high probability that a desired microstructure is notobtained. Therefore, it is preferable that the steel sheet is cooled atan average rate of 1° C./s or more in a temperature range of T° C. orhigher and 550° C. or lower.

The steel sheet cooled to the cooling termination temperature: T° C. iskept in the first temperature range of 350° C. or higher and 490° C. orlower for a period of 15 seconds or more, and 1,000 seconds or less. Ifthe upper limit of the first temperature range exceeds 490° C., carbidesare deposited from the untransformed austenite and, thereby, a desiredmicrostructure is not obtained. On the other hand, in the case where thelower limit of the first temperature range is lower than 350° C., aproblem occurs in that lower bainite is generated rather than upperbainite and the amount of C concentrated into austenite is reduced.Therefore, the first temperature range is 350° C. or higher and 490° C.or lower. Preferably, the range is 370° C. or higher and 460° C. orlower.

Moreover, in the case where the maintenance time in the firsttemperature range is less than 15 seconds, a problem occurs in that theamount of upper bainite transformation is reduced and the amount of Cconcentrated into untransformed austenite is reduced. On the other hand,in the case where the maintenance time in the first temperature rangeexceeds 1,000 seconds, carbides are deposited from untransformedaustenite which serves as retained austenite in the final microstructureof the steel sheet, stable retained austenite, into which C has beenconcentrated, is not obtained and, as a result, a desired workability isnot obtained. Therefore, the maintenance time is 15 seconds or more and1,000 seconds or less. Preferably, the range is 30 seconds or more and600 seconds or less.

After maintaining the first temperature range is completed, theresulting steel sheet is cooled to a second temperature range of 200° C.or higher and 350° C. or lower at any rate and is kept in the secondtemperature range for a period of 15 seconds or more and 1,000 secondsor less. If the upper limit of the second temperature range exceeds 350°C., a problem occurs in that lower bainite transformation does notproceed and, as a result, the amount of as-quenched martensiteincreases. On the other hand, in the case where the lower limit of thesecond temperature range is lower than 200° C. as well, a problem occursin that lower bainite transformation does not proceed and the amount ofas-quenched martensite increases. Therefore, the second temperaturerange is 200° C. or higher and 350° C. or lower. Preferably, the rangeis 250° C. or higher and 340° C. or lower.

Moreover, in the case where the maintenance time is less than 15seconds, an adequate amount of lower bainite is not obtained, anddesired workability is not obtained. On the other hand, in the casewhere the maintenance time exceeds 1,000 seconds, carbides are depositedfrom the stable retained austenite in the upper bainite generated in thefirst temperature range and, as a result, desired workability is notobtained. Therefore, the maintenance time is 15 seconds or more and1,000 seconds or less. Preferably, the range is 30 seconds or more and600 seconds or less.

In this regard, in a series of heat treatments, the maintenancetemperature is not necessarily a constant insofar as the maintenancetemperature is within the above-described predetermined temperaturerange, and fluctuation within a predetermined temperature range does notimpair the steel sheets. The same goes for the cooling rate.Furthermore, the steel sheet may be heat-treated with any facilityinsofar as only the thermal history is satisfied. In addition, temperrolling may be applied to the surface of the steel sheet or a surfacetreatment, e.g., electroplating, may be applied after the heat treatmentto correct the shape.

The method for manufacturing a high strength steel sheet can furtherinclude a galvanizing treatment or a galvannealing treatment in which analloying treatment is further added to the galvanizing treatment. Thegalvanizing treatment or, furthermore, the galvannealing treatment maybe conducted during the above-described cooling to the first temperaturerange or in the first temperature range. In this case, the maintenancetime in the first temperature range is 15 seconds or more and 1,000seconds or less, in which a treatment time of the galvanizing treatmentor the galvannealing treatment in the first temperature range isincluded. In this connection, it is preferable that the galvanizingtreatment or the galvannealing treatment is conducted with a continuousgalvanizing and galvannealing line.

Furthermore, the method for manufacturing a high strength steel sheetcan include that the high strength steel sheet is produced following theabove-described manufacturing method where steps up to the heattreatment have been completed and, thereafter, the galvanizing treatmentor, furthermore, the galvannealing treatment is conducted.

Alternatively, after maintaining the second temperature range followingthe manufacturing method, the galvanizing treatment or the galvannealingtreatment can be conducted succeedingly.

A method for applying a galvanizing treatment or a galvannealingtreatment to a steel sheet is as described below.

The steel sheet is immersed into a plating bath, and the amount ofadhesion is adjusted through gas wiping or the like. It is preferablethat the amount of Al dissolved in the plating bath is 0.12% or more and0.22% or less in the case of the galvanizing treatment and 0.08% or moreand 0.18% or less in the case of the galvannealing treatment.

Regarding the treatment temperature, as for the galvanizing treatment,the temperature of the plating bath may be 450° C. or higher and 500° C.or lower and, furthermore, in the case where the galvannealing treatmentis applied, it is preferable that the temperature during alloying is550° C. or lower. In the case where the alloying temperature exceeds550° C., carbides are deposited from untransformed austenite and in somecases, pearlite is generated. Consequently, the strength or theworkability, or the two are not obtained. In addition, the powderingproperty of the coating layer deteriorates. On the other hand, if thetemperature during alloying is lower than 450° C., in some cases,alloying does not proceed. Therefore, it is preferable that the alloyingtemperature is 450° C. or higher.

It is preferable that the coating mass is 20 g/m² or more and 150 g/m²or less per surface. If the coating mass is less than 20 g/m², thecorrosion resistance becomes inadequate. On the other hand, even when150 g/m² is exceeded, the corrosion-resisting effect is saturated and anincrease in the cost is likely.

It is preferable that the degree of alloying of the coating layer (Fepercent by mass (Fe content)) is 7 percent by mass or more and 15percent by mass or less. If the degree of alloying of the coating layeris less than 7 percent by mass, alloying variations occur so that thequality of outward appearance deteriorates, or a so-called a ζ phase isgenerated in the coating layer so that the sliding property of the steelsheet deteriorates. On the other hand, if the degree of alloying of thecoating layer exceeds 15 percent by mass, large amounts of hard brittleΓ phase is formed so that the adhesion of the coating deteriorates.

EXAMPLES

Our steel sheets and methods will be described below in further detailwith reference to the examples. However, the following examples do notlimit the scope of this disclosure. In this connection, modification ofthe configurations and range is included in the scope of thisdisclosure.

An ingot obtained by melting a steel having a component compositionshown in Table 1 was heated to 1,200° C. and subjected to finish hotrolling at 870° C. The resulting hot-rolled steel sheet was taken up at650° C. and, subsequently, the hot-rolled steel sheet was pickled.Thereafter, cold rolling was conducted at a reduction ratio of 65% toproduce a cold-rolled steel sheet having a sheet thickness: 1.2 mm. Theresulting cold-rolled steel sheet was subjected to a heat treatmentunder the conditions shown in Table 2. In this connection, the coolingtermination temperature: T in Table 2 refers to a temperature at whichcooling of a steel sheet is terminated in cooling of the steel sheetfrom the annealing temperature.

Furthermore, a part of cold-rolled steel sheets were subjected to agalvanizing treatment or a galvannealing treatment. As for thegalvanizing treatment, plating was conducted on both surfaces at aplating bath temperature: 463° C. in such a way that a mass per unitarea (per surface): 50 g/m² was ensured. Moreover, as for thegalvannealing treatment, plating was conducted on both surfaces whilethe alloying condition was adjusted in such a way that a mass per unitarea (per surface): 50 g/m² was ensured and the degree of alloying (Fepercent by mass (Fe content)) became 9 percent by mass. The galvanizingtreatment and the galvannealing treatment were conducted after coolingwas once conducted to T° C. shown in Table 2.

The resulting steel sheet was subjected to temper rolling at a reductionratio (elongation percentage): 0.3 after a heat treatment in the casewhere a plating treatment is not conducted, or after a galvanizingtreatment or a galvannealing treatment in the case where thesetreatments were conducted.

TABLE 1 Steel type C Si Mn Al P S N Cr V Mo Ti Nb B A 0.311 1.96 1.540.041 0.009 0.0024 0.0025 — — — — — — B 0.299 1.98 1.99 0.042 0.0130.0019 0.0034 — — — — — — C 0.305 2.52 2.03 0.043 0.010 0.0037 0.0042 —— — — — — D 0.413 2.03 1.51 0.038 0.012 0.0017 0.0025 — — — — — — E0.417 1.99 2.02 0.044 0.010 0.0020 0.0029 — — — — — — F 0.330 1.45 2.820.040 0.012 0.0031 0.0043 — — — — — — G 0.185 1.52 2.32 0.048 0.0200.0050 0.0044 — — — — — — H 0.522 1.85 1.48 0.040 0.011 0.0028 0.0043 —— — — — — I 0.320 0.99 2.25 0.041 0.014 0.0018 0.0042 — — — — — — J0.263 1.50 2.29 0.039 0.011 0.0010 0.0036 0.9 — — — — — K 0.270 1.352.27 0.043 0.004 0.0020 0.0035 — 0.21 — — — — L 0.221 1.22 1.99 0.0400.040 0.0030 0.0043 — — 0.19 — — — M 0.202 1.75 2.52 0.045 0.044 0.00200.0044 — — — 0.035 — — N 0.175 1.51 2.18 0.042 0.022 0.0020 0.0044 — — —— 0.07 — O 0.212 1.51 2.37 0.043 0.030 0.0010 0.0029 — — — 0.020 —0.0011 P 0.480 1.52 1.33 0.044 0.015 0.0020 0.0038 — — — — — — Q 0.3101.42 2.02 0.043 0.015 0.0030 0.0023 — — — — — — R 0.335 2.01 2.22 0.0430.004 0.0028 0.0041 — — — — — — S 0.329 1.88 1.65 0.040 0.021 0.00200.0031 — — — — — — T 0.330 0.01 2.33 1.010 0.025 0.0020 0.0033 — — — — —— U 0.291 — 2.75 0.042 0.012 0.0040 0.0024 — — — — — — V 0.290 0.48 2.220.130 0.006 0.0020 0.0035 — — — — — — W 0.145 0.50 1.42 0.320 0.0070.0018 0.0041 — — — — — — X 0.190 1.00 0.41 0.036 0.013 0.0020 0.0038 —— — — — — Steel A₃point type Ni Cu Ca REM Si + Al (° C.) Remarks A — — —— 2.00 850 Steel B — — — — 2.02 842 Steel C — — — — 2.56 862 Steel D — —— — 2.07 838 Steel E — — — — 2.03 820 Steel F — — — — 1.49 787 Steel G —— — — 1.57 841 Steel H — — — — 1.89 815 Steel I — — — — 1.03 787 Steel J— — — — 1.54 807 Steel K — — — — 1.39 827 Steel L — — — — 1.26 849 SteelM — — — — 1.80 872 Steel N — — — — 1.55 848 Steel O — — — — 1.55 848Steel P 0.52 — — — 1.56 806 Steel Q — 0.55 — — 1.46 805 Steel R — —0.003 — 2.05 824 Steel S — — — 0.002 1.92 848 Steel T — — — — 1.02 873Steel U — — — — 0.04 732 Comparative Steel V — — — — 0.61 111Comparative Steel W — — — — 0.82 859 Comparative Steel X — — — — 1.04868 Comparative Steel Note) Underline indicates that the value is out ofthe appropriate range.

TABLE 2 Average cooling Cooling rate Annealing Annealing rate to 550° C.to Sample Steel Coating temperature time 550° C. T° C. No. type *2 (°C.) (s) (° C./s) (° C./s) 1 A CR 880 180  4 15 2 A CR 900 180 20 20 3 ACR 900 200 50 50 4 A CR 900 200 50 50 5 B CR 800 200 20 20 6 B CR 880200 20 20 7 B CR 880 350 35 35 8 C CR 890 150 25 25 9 C CR 900 200 20 2010 D CR 900 200 20 20 11 D CR 900 200 50 50 12 E CR 880 250 15 15 13 FCR 870 300 20 20 14 F GI 870 300 12 12 15 G CR 890 200 20 20 16 H CR 880200 25 25 17 I CR 900 250 30 30 18 I GA 900 250 20 20 19 J CR 900 200 2020 20 K CR 900 200 40 40 21 L CR 900 200 30 30 22 M CR 900 200 20 20 23N CR 900 200 20 20 24 O CR 900 200 20 20 25 P CR 900 200 20 20 26 Q CR900 200 30 30 27 R CR 900 200 30 30 28 S CR 900 200 30 30 29 T CR 900200 30 30 30 U CR 900 200 13 13 31 V CR 900 200 20 20 32 W CR 900 200 4040 33 X CR 900 200 15 15 Maintaining Second temperature Cooling time inrange termination first Maintaining Maintaining Sample temperaturetemperature temperature times No. (° C.) range (s) (° C.) (s) Remarks 1430 100 300 100 Comparative Example 2 400  5 320  90 Comparative Example3 420 100 330 180 Example 4 400 100 330 300 Example 5 400 120 300 100Comparative Example 6 520 200 330 300 Comparative Example 7 400 100 330350 Example 8 400  80 110 120 Comparative Example 9 380 120 310 300Example 10 400 100 330 300 Example 11 400 300 250  10 ComparativeExample 12 400 200 340 550 Example 13 450 100 330 250 Example 14 450 100330 200 Example 15 400  90 240 420 Example 16 370 400 200 500 Example 17400 150 250 300 Example 18 450 100 280 100 Example 19 370  90 300 300Example 20 420  90 300 300 Example 21 420 200 300 300 Example 22 420 180300 300 Example 23 420 100 300 300 Example 24 420 100 300 300 Example 25420 300 300 300 Example 26 420 120 300 300 Example 27 420 100 300 300Example 28 420 100 300 300 Example 29 420 120 300 300 Example 30 420 100300 300 Comparative Example 31 420 100 300 300 Comparative Example 32420  60 300 300 Comparative Example 33 420  60 300 300 ComparativeExample *1 Underline indicates that the value is out of the appropriaterange. *2 CR: No coating (cold-rolled steel sheet) GI: Galvanized steelsheet GA :Galvannealed steel sheet

Various characteristics of the thus obtained steel sheet were evaluatedby the following methods.

A sample was cut from each steel sheet and was polished. Microstructuresof ten fields of view of a surface parallel to the rolling directionwere observed with a scanning electron microscope (SEM) at 3,000-foldmagnification, the area percentage of each phase was measured, and aphase structure of each crystal grain was identified.

The steel sheet was ground•polished up to one-quarter of a sheetthickness in the sheet thickness direction and the amount of retainedaustenite was determined by X-ray diffractometry. As for an incidentX-ray, Co—Kα was used and the amount of retained austenite werecalculated from the average value of the intensity ratio of each of(200), (220), and (311) faces of austenite to the diffraction intensityof each of (200), (211), and (220) faces of ferrite.

As for the average amount of C in the retained austenite, a latticeconstant was determined from the intensity peak of each of (200), (220),and (311) faces of austenite based on the X-ray diffractometry, and theaverage amount of C (percent by mass) in the retained austenite wasdetermined from the following calculation formula:

a0=0.3580+0.0033×[C %]+0.00095×[Mn %]+0.0056×[Al %]+0.022×[N %]

where, a0 represents a lattice constant (nm) and [X %] representspercent by mass of an element X. The percent by mass of an element otherthan C was percent by mass relative to whole steel sheet.

The tensile test was conducted based on JIS Z2241 by using a test pieceof JIS No. 5 size taken in a direction perpendicular to the rollingdirection of the steel sheet. The TS (tensile strength) and the T.E(total elongation) were measured, a product of the strength and thetotal elongation (TS×T.El) was calculated and, thereby, the balancebetween the strength and the workability (elongation) was evaluated.Cases where TS×T.El≧20,000 MPa·% were evaluated as “good.”

The stretch-flangeability was evaluated on the basis of the Japan Ironand Steel Federation Standard JFST 1001. Each of the resulting steelsheets was cut into 100 mm×100 mm, a hole having a diameter: 10 mm waspunched with a clearance of 12% of sheet thickness. Thereafter, a dicehaving an inside diameter: 75 mm was used, a 60° circular cone punch waspushed into the hole while holding was conducted with a holddown force:88.2 kN, a hole diameter at crack occurrence limit was measured, and ahole-expansion limit λ (%) was determined from the formula (1):

hole-expansion limit λ(%)={(Df−D0)/D0}×100   (1)

where Df represents a hole diameter (mm) at occurrence of crack and D0represents an initial hole diameter (mm).

The thus measured λ was used, the product of the strength and thehole-expansion limit (TS×2) was calculated and, thereby, the balancebetween the strength and the stretch-flangeability was evaluated.

Stretch-flangeability was evaluated as “good” in the case whereTS×λ≧25,000 MPa·%.

Furthermore, the hardness of the hardest microstructure in the steelsheet micro-structure was determined by a method described below. Thatis, as a result of microstructure observation, in the case whereas-quenched martensite was observed, 10 points of the as-quenchedmartensite were measured with an ultramicro-Vickers at a load: 0.02 N,and an average value thereof was assumed to be the hardness of thehardest microstructure in the steel sheet microstructure. In thisconnection, in the case where as-quenched martensite is not observed, asdescribed above, the microstructure of any one of the temperedmartensite, the upper bainite, and the lower bainite becomes the hardestphase in our steel sheets. In the case of our steel sheets, the hardestphase was a phase showing HV ≦800.

The above-described evaluation results are shown in Table 3.

TABLE 3 (As- Area percentage relative to whole steel sheet microstructure (%) quenched Sample Steel LB*² + As- αb + LB + M)/(M + No.type αb*² M*² quenched M α*² γ*²*³ Remainder M + γ LB) (%) 1 A  3  6 058  1 32   10  0 2 A  4 89 10 3 4 0  97 11 3 A 54 31 10 2 13  0  98 32 4A 56 30 7 2 12  0  98 23 5 B 21 49 10 21  6 3  76 20 6 B 37 49 10 3 8 3 94 20 7 B 50 38 10 0 12  0 100 26 8 C 50 35 28 0 15  0 100 80 9 C 52 3411 0 14  0 100 32 10 D 45 39 9 0 16  0 100 23 11 D 58 21 18 0 21  0 10086 12 E 25 63 30 0 12  0 100 48 13 F 15 78 30 0 7 0 100 38 14 F 14 76 302 8 0  98 39 15 G 70 14 2 7 9 0  93 14 16 H 16 78 17 0 6 0 100 22 17 I37 52 20 0 11  0 100 38 18 I 36 55 38 0 9 0 100 69 19 J 16 75 14 1 9 0100 19 20 K 22 69 21 0 9 0 100 30 21 L 20 69 22 0 11  0 100 32 22 M 3656 13 0 8 0 100 23 23 N 33 58 35 0 9 0 100 60 24 O 35 55 15 0 10  0 10027 25 P 30 57 25 0 13  0 100 44 26 Q 40 44 18 0 16  0 100 41 27 R 22 6817 0 10  0 100 25 28 S 60 29 12 0 11  0 100 41 29 T 42 47 25 0 11  0 10053 30 U 40 41 20 9 2 8  83 49 31 V 39 54 24 4 3 0  96 44 32 W 78  8 3 03 11   89 38 33 X  8  1 1 70  0 21   9 100  Average amount of C inSample retained TS T.EL λ TS × T.EL TS × λ No. γ (%) (MPa) (%) (%) (MPa· %) (MPa · %) Remarks 1 —  841 21 38 17661 31958 Comparative Example 20.91 1492 12 20 17904 29840 Comparative Example 3 1.14 1166 19 34 2215439644 Example 4 1.23 1156 21 34 24276 39304 Example 5 0.68 1296 13 2016848 25920 Comparative Example 6 0.57 1467 11 22 16137 32274Comparative Example 7 1.22 1302 18 23 23436 29946 Example 8 0.94 1482 205 29640 7410 Comparative Example 9 1.16 1371 19 20 26049 27420 Example10 1.36 1335 24 21 32040 28035 Example 11 1.20 1203 29 8 34887 9624Comparative Example 12 1.15 1695 15 18 25425 30510 Example 13 0.81 171014 19 23940 32490 Example 14 0.75 1632 13 19 21216 31008 Example 15 0.741098 21 45 23058 49410 Example 16 1.09 1820 14 18 25480 32760 Example 170.85 1395 16 21 22320 29295 Example 18 0.82 1314 17 20 22338 26280Example 19 0.81 1783 12 17 21396 30311 Example 20 0.83 1612 13 19 2095630628 Example 21 0.73 1870 11 14 20570 26180 Example 22 0.82 1285 21 2026985 25700 Example 23 0.79 1045 25 38 26125 39710 Example 24 0.86 123019 28 23370 34440 Example 25 0.92 1771 14 19 24794 33649 Example 26 0.951596 13 20 20748 31920 Example 27 0.96 1482 14 37 20748 54834 Example 281.05 1465 17 28 24905 41020 Example 29 1.07 1355 19 33 25745 44715Example 30 — 1183 13 23 15379 27209 Comparative Example 31 — 1288 12 2315456 29624 Comparative Example 32 —  901 14 32 12614 28832 ComparativeExample 33 —  735 14 30 10290 22050 Comparative Example *¹Underlineindicates that the value is out of the appropriate range. *²αb: Bainiticferrite in upper bainite LB: Lower bainite M: Martensite α: Polygonalferrite γ: Retained austenite *³Amount of retained austenite determinedby X-ray diffractometry was assumed to be area percentage relative towhole steel sheet microstructure.

As is clear from Table 3, our steel sheets satisfy that the tensilestrength is 980 MPa or more, the value of TS×T.El is 20,000 MPa·% ormore, and TS×λ≧25,000 MPa·%. Therefore, it was able to be ascertainedthat high strength and excellent workability, especially excellentstretch-flangeability, were provided in combination.

On the other hand, regarding Sample No. 1, the average cooling rate to550° C. was out of the appropriate range. Therefore, a desired steelsheet microstructure was not obtained. Although TS×λ≧25,000 MPa·% wassatisfied, the tensile strength (TS) ≧980 MPa and TS×T.El≧20,000 MPa·%were not satisfied. Regarding Sample No. 2, the maintenance time in thefirst temperature range was out of the appropriate range. RegardingSample No. 5, the annealing temperature was lower than A₃ point.Regarding Sample No. 6, the cooling termination temperature: T was outof the first temperature range. Regarding Sample No. 8, the maintenancetemperature in the second temperature range was out of the appropriaterange. Regarding Sample No. 11, the maintenance time in the secondtemperature range was out of the appropriate range. Therefore, a desiredsteel sheet microstructure was not obtained. Although the tensilestrength (TS) ≧980 MPa was satisfied, any one of TS×T.El≧20,000 MPa·%and TS×λ≧25,000 MPa·% was not satisfied. Regarding Sample Nos. 30 to 34,the component compositions were out of the appropriate range. Therefore,a desired steel sheet microstructure was not obtained, and at least oneof the tensile strength (TS) ≧980 MPa, TS×T.El≧20,000 MPa·%, andTS×λ≧25,000 MPa·% was not satisfied.

1. A high strength steel sheet having a composition comprising, on a percent by mass basis, C: 0.17% to 0.73%; Si: 3.0% or less; Mn: 0.5% to 3.0%; P: 0.1% or less; S: 0.07% or less; Al: 3.0% or less; and N: 0.010% or less, satisfies Si+Al≧0.7%, and the remainder includes Fe and incidental impurities, with a microstructure, it is satisfied that the that has an area percentage of a total amount of lower bainite and whole martensite 10% to 90% relative to the whole steel sheet microstructure, an amount of retained austenite is 5% to 50%, an area percentage of bainitic ferrite in upper bainite is 5% or more relative to the whole steel sheet microstructure, as-quenched martensite is 75% or less of the total amount of lower bainite and whole martensite, and an area percentage of polygonal ferrite is 10% or less relative to the whole steel sheet microstructure, an average amount of C in retained austenite is 0.70% or more, and tensile strength is 980 MPa or more.
 2. The high strength steel sheet according to claim 1, further comprising at least one element selected from the group consisting of, on a percent by mass basis, Cr: 0.05% to 5.0%; V: 0.005% to 1.0%; and Mo: 0.005% to 0.5%.
 3. The high strength steel sheet according to claim 1, further comprising at least one element selected from the group consisting of, on a percent by mass basis, Ti: 0.01% to 0.1%; and Nb: 0.01% to 0.1%.
 4. The high strength steel sheet according to claim 1, further comprising, on a percent by mass basis, B: 0.0003% to 0.0050%.
 5. The high strength steel sheet according to claim 1, further comprising at least one element selected from the group consisting of, on a percent by mass basis, Ni: 0.05% to 2.0%; and Cu: 0.05% to 2.0%.
 6. The high strength steel sheet according to claim 1, further comprising at least one element selected from the group consisting of, on a percent by mass basis, Ca: 0.001% to 0.005%; and REM: 0.001% to 0.005%.
 7. A high strength steel sheet comprising a galvanized layer or a galvannealed layer on a surface of the steel sheet according to claim
 1. 8. A method for manufacturing a high strength steel sheet comprising: hot-rolling a billet having a component composition according to claim 1; conducting cold-rolling to produce a cold-rolled steel sheet; annealing the resulting cold-rolled steel sheet for 15 to 600 seconds in an austenite single phase region; and conducting cooling to a cooling termination temperature: T° C. determined in a first temperature range of 350° C. to 490° C., wherein cooling to at least 550° C. is conducted while an average cooling rate is controlled at 5° C./s or more, subsequently, temperature is maintained in the first temperature range for 15 to 1,000 seconds and, then, temperature is maintained in a second temperature range of 200° C. to 350° C. for 15 to 1,000 seconds or less.
 9. The method according to claim 8, further comprising applying galvanizing treatment or a galvannealing treatment during cooling to a cooling termination temperature: 1° C. or in the first temperature range.
 10. The high strength steel sheet according to claim 2, further comprising at least one element selected from the group consisting of, on a percent by mass basis, Ti: 0.01% to 0.1%; and Nb: 0.01% to 0.1%.
 11. The high strength steel sheet according to claim 2, further comprising, on a percent by mass basis, B: 0.0003% to 0.0050%.
 12. The high strength steel sheet according to claim 3, further comprising, on a percent by mass basis, B: 0.0003% to 0.0050%.
 13. The high strength steel sheet according to claim 2, further comprising at least one element selected from the group consisting of, on a percent by mass basis, Ni: 0.05% to 2.0%; and Cu: 0.05% to 2.0%.
 14. The high strength steel sheet according to claim 3, further comprising at least one element selected from the group consisting of, on a percent by mass basis, Ni: 0.05% to 2.0%; and Cu: 0.05% to 2.0%.
 15. The high strength steel sheet according to claim 4, further comprising at least one element selected from the group consisting of, on a percent by mass basis, Ni: 0.05% to 2.0%; and Cu: 0.05% to 2.0%.
 16. The high strength steel sheet according to claim 2, further comprising at least one element selected from the group consisting of, on a percent by mass basis, Ca: 0.001% to 0.005%; and REM: 0.001% to 0.005%.
 17. The high strength steel sheet according to claim 3, further comprising at least one element selected from the group consisting of, on a percent by mass basis, Ca: 0.001% to 0.005%; and REM: 0.001% to 0.005%.
 18. The high strength steel sheet according to claim 4, further comprising at least one element selected from the group consisting of, on a percent by mass basis, Ca: 0.001% to 0.005%; and REM: 0.001% to 0.005%.
 19. The high strength steel sheet according to claim 5, further comprising at least one element selected from the group consisting of, on a percent by mass basis, Ca: 0.001% to 0.005%; and REM: 0.001% to 0.005%. 